The invention relates to a control system of the kind as described in the preamble of claim 1 and as known as such from EP-A-0 423 536.
In this known transmission system the outlet port of the supply pump which supplies the hydraulic medium under pressure from a medium supply is connected via two one-way valves with the respective inlet ports of each of the actuators, which are thus connected with each other only via the first hydraulic pump-motor and from which no hydraulic medium can flow back to the supply.
This known transmission has two principle drawbacks. The first drawback is that this transmission can only work in a very narrow range of transmission ratios at both sides of the transmission ratio: one. This results from the fact that, when the transmission ratio deviates largely from the value one, thus when the running radius of the transmission medium on the one of the pair of conical disks is small, and the same on the other pair large, the adjustment of the conical disksxe2x80x94in which the disks with the smaller running radius move towards each other and same with the greater running radius move away from each otherxe2x80x94the distance over which the disks must be adjusted with respect to each other in axial direction is unequal for the respective pairs of conical disks; of course then also the respective amounts of displaced hydraulic medium are unequal. In this know device, however, as a result of the one-way valves no hydraulic medium can flow back to the supply volume so that the control system will inevitably lock itself up when such adjustments take place.
A second drawback is the fact that the second hydraulic supply pump is a simple supply pump combined with a spring biased shunt valve, the pump being driven constantly. As long as the transmission ratio does not change all the energy supplied by the pump is converted in the bias valve in useless heat. Thus the pump provokes very great energy losses which greatly reduce the total efficiency of the transmission.
The aim of the invention is to improve a control system of the kind as mentioned above in such a way that it can be operated over a very large range of ratios with a minimum of losses. This aim is obtained with the measures according to the characterizing part of claim 1.
As a result of the open connection between the outlet port of the two-quadrants controlled second pump-motor and the inlet port of one of the actuators medium can be supplied from, and returned to, the supply volume, ensuring that the in four-quadrants controlled, first pump-motor can cover without any problems the whole range of ratios of the transmission, while the fact that the second pump-motor can be controlled in two quadrants results in that this second pump-motor must only set a minimum pressure level for one of the actuators, which has a value only a little above the value with which under the present transmission ratio and torque conditions slipping of the transmission means would occur, the volume flow which must be supplied is very small, not more than necessary to compensate the difference in volume flow from the one actuator to the other with added to it the internal and external leak losses which might occur in the transmission or in the control system. When the transmission ratio remains constant this volume flow remains restricted to only the amount which is necessary to compensate occurring leak losses. As a result the energy consumed by the second pump-motor will only be very small and the total efficiency of the transmission will be increased in proportion thereto.
It is observed that the measures according to the invention result into a transmission of which the control and supply system is made up with simple, relatively cheap and compact, electric motors which, with the present prior art can be operated and controlled quite dependable. It is not necessary any more to use the known hydraulic pump, driven by the engine which drives the transmission and which provokes quite heavy losses.
The preferred embodiment according to claim 5 has the advantage that the second pump-motor will always be connected to the actuator which carries the lowest hydraulic setting pressure so that this second pump-motor, irrespective of the direction in which the energy flow passes through the transmission and irrespective of the actual transmission ratio, needs only to supply the lowest pressure level, thus resulting in a further reduction of energy taken up by this pump-motor.